Microporous organic polymers have high surface areas and as such are useful in numerous areas such as separations, controlled release, gas storage and supports for catalysts.
There are only a handful of ways to synthesize microporous organic polymers.
One method is to form hypercrosslinked polymers (HCPs) (Lee et al. Chem. Commun. 2006, 2670) but the degree of direct synthetic control over average pore size is limited and the materials are not conjugated.
A second method is to form “polymers of intrinsic microporosity” (PIMs) (Budd et al. Chem. Commun. 2004, 230); these are rigid, contorted polymers which display permanent microporosity. Again, there is no direct synthetic control over pore size and the materials are not conjugated.
A third method is to produce covalent organic frameworks (COFs) (Cote et al. Science 2005, 310, 1166) by condensation reactions of 1,4-benzenediboronic acid (BDBA). The materials, however, are also not conjugated and it is difficult to see how they could be made so given the boronate chemistry used. The materials are crystalline (ordered).
A fourth method is to form a conjugated polymer and then to pyrolyze this at high temperatures (M. Kijima et al., Carbon 2007 (45), 594-601). This method leads to microporosity and conjugation but does not allow direct synthetic control over pore size.
From a first aspect the present invention provides a method for preparing a conjugated microporous polymer comprising the coupling of:
an alkynyl aryl monomer having a plurality of terminal alkyne groups, with
an iodo- or bromo-aryl monomer having a plurality of halogen atoms, in the presence of a palladium (0) catalyst.
The term “conjugated microporous polymer” is in places abbreviated herein to “CMP”.
Thus, during the above-mentioned coupling, the bond to the iodo or bromo group in the iodo- or bromo-aryl monomer is replaced by a bond to the terminal alkyne portion of the alkynyl aryl monomer.
Because the alkynyl aryl monomer has a plurality of terminal alkyne groups it is able to react with more than one iodo- or bromo-aryl monomer. Similarly, because the iodo- or bromo-aryl monomer has a plurality of halogen atoms it is able to react with more than one alkynyl aryl monomer. In this way a polymer network comprising aryl units and alkyne units is formed.
Some of the aryl units act as, or form part of, nodes; and the alkyne units act as, or form part of, struts connecting the nodes. Thus the method is highly advantageous in allowing the geometry and structure of the polymer networks to be controlled and tailored according to particular requirements. In particular, variation of the components can be carried out in order to effect different micropore sizes, pore volumes, and surface areas.
The alkyne units result in linear rigid linkages and therefore variation of the length of the struts allows the nodes to be held apart by particular predetermined distances. Nevertheless, the skilled person will understand that, just as in other areas of chemistry, the linearity and rigidity of the linkages are not absolute and deviations from true linearity can and do occur, so that various polymeric forms are possible, including for example sheets, amorphous structures, nets, loops, interpenetrated or “catenated” structures, cylinders and spheres. This variation of polymeric forms is not inconsistent with the reproducible preparation of products with particular properties, which reproducibility is shown below in the examples.
The present inventors have found that the use of a palladium (0) catalyst allows the polymer networks to be formed in high yield and results in polymers having high surface areas. In contrast, as shown below in a comparative example, the use of a palladium (II) catalyst was found to result in a product having higher residual halogen content and lower surface area. Higher residual halogen content is an indication that less coupling has taken place and is disadvantageous in resulting in less predictable geometric structures, greater cost, heavier products and adverse environmental and chemical effects.
A catalytic amount of a co-catalyst, for example copper iodide, may optionally be used to enhance the coupling as known to those skilled in the art.
Iodo-aryl monomers are generally more reactive than bromo-aryl monomers and therefore are preferable. Nevertheless, bromo-aryl monomers function well and are perfectly acceptable for use in the present invention.
The aryl component in either or both of the alkynyl aryl monomer and/or the iodo- or bromo-aryl monomer may be either aromatic or heteroaromatic, and may optionally contain more than one aromatic or heteroaromatic ring. Possible aromatic rings include for example benzene, naphthalene and anthracene. Other possible rings include for example pyridines, carbazol, phenyl groups bearing methyl or other groups, aniline, or rings with pendant amine groups. The present invention is thus compatible with the use of unsubstituted or substituted aromatic or heteroaromatic systems.
The aryl component in either or both of the alkynyl aryl monomer and/or the iodo- or bromo-aryl monomer may optionally carry one or more substituent. Optionally at least one of said substituents may be for example amino, alkyl (e.g. methyl or other C1-5 alkyl), haloalkyl (e.g. trifluoromethyl), an azide, fluorine, alkenyl (e.g. C1-5 alkenyl), hydroxyl, thiol, ester, amide, urethane, carbonate, acetate, ether, thioether, oligomer, or more complex chemical moiety (e.g., catalyst fragment, amino acid); furthermore said substituents may themselves be further optionally substituted.
The reagents and products may contain aryl units in various configurations and linked to various units. For example, aryl units (e.g. three phenyl rings) be bound to a central nitrogen atom; or porphyrin rings may be present; or aryl units (e.g. four phenyl rings) may be bound to a central silicon atom.
The alkynyl aryl monomer comprises at least one alkyne bond in conjugation with at least one aryl component.
The rigidity of the alkyne bond is in effect extended as required through as many conjugated linkages as are required.
By way of example, a relatively simple conjugated strut may comprise two alkyne bonds and a benzene ring as follows:

A longer conjugated strut could for example be:

Nevertheless, the alkynyl aryl monomer does not necessarily need to contain exclusively alternating aryl and alkynyl structures. For example the following is an example of a medium-length strut:

The aryl-containing nodes may be derived either from the alkynyl aryl monomer or from the iodo- or bromo-aryl monomer. Likewise, aryl portions linked to the alkyne bonds in the struts may be derived from either of the starting materials. For example, the combination of monomers in either row of the following table (wherein Hal denotes I or Br):—
Alkynyl arylBromo-aryl or iodo-arylstarting materialstarting materialCom- bi- na- tion 1 Com- bi- na- tion 2may be used to make the following network, shown schematically:—

In the above-mentioned example the nodes are benzene rings:—
whether deriving from

However, the node may comprise more than one aryl ring and/or the points of attachment to the node may be on different rings. Further examples of suitable nodes are:

For some applications functionalized polymers are useful. Therefore in one preference at least one of the aryl groups in the alkynyl aryl monomer and the iodo- or bromo-aryl monomer is not unsubstituted benzene. Surprisingly the reaction works well with substituted or functionalized starting materials and this functionality is useful in allowing further reactions and subsequent derivatization to take place.
Preferably either the alkynyl aryl monomer has at least three terminal alkyne groups or the iodo- or bromo aryl monomer has at least three halogen atoms.
More preferably,
either:—
the alkynyl aryl monomer has three terminal alkyne groups and the iodo- or bromo-aryl monomer has two or three (most preferably two) halogen atoms;
or:—
the alkynyl aryl monomer has two or three (most preferably two) terminal alkyne groups and the iodo-bromo-aryl monomer has three halogen atoms.
This results in preferred polymers wherein the aryl-containing hubs are trifunctional, i.e. wherein a hub is linked to three struts. This is geometrically favourable and allows the formation of networks with useful pore sizes and specific surface areas.
Preferably the struts are approximately or exactly equally spaced around a hub. Thus for example a hub may be a 1,3,5-substituted benzene ring amongst many other possibilities.
In other examples the hub may have tetra or higher functionality in which case one of the monomers has tetra or higher functionality, i.e. four or more terminal alkyne groups or halogen atoms.
The monomers and their valency may be varied and chosen so that the present invention allows a flexible approach and the tailoring of polymer networks. In a further alternative, both monomers may be trifunctional. In a yet further alternative, more than one type of alkynyl aryl monomer and/or more than one type of halo aryl monomer may be used: for example a copolymer may be made when for example the monomers are an alkynyl aryl monomer, a first halo aryl monomer, and a second halo aryl monomer.
The ratio of functionalities in the reagents (for example the ratio of ethynyl to halogen functionalities) may optionally be 1.5 to 1.
Preferably the struts are symmetrical. The struts preferably contain at least two alkyne units, each of which are bound to the hub via an aryl unit, and between the alkyne units there is at least one aryl unit in the struts.
Optionally the reaction may be carried out in the present of a solvent. Such a solvent should be inert under the conditions used. For example, toluene is suitable whereas alcohols or water are not.
The reaction is carried out in the present of a base. The base may be an organic base such as a tertiary or secondary aliphatic amine for example triethylamine, diethylamine, or diisopropylamine.
The reaction medium may for example comprise both toluene and triethylamine, for example in a ratio of 1:5 to 5:1, for example 1:2 to 2:1, for example approximately 1:1, volume:volume ratio.
In some prior art processes there are alkene or allene linkages either present in the starting materials or produced during reaction to the polymer product. This results in cis/trans isomers and lower predictability in terms of geometric structure. In the present invention, alkyne bonds are present in the starting materials and are conserved in the polymer product. This allows polymers of predictable structure to be formed. In the present invention the majority of the unsaturated linkages in the polymer network are not alkene or allene linkages. Preferably the polymer networks of the present invention are substantially free from alkene or allene linkages.
Alkyne-containing polymers have different properties to alkene- or allene-containing polymer or polymers containing saturated linkages. Chemically, alkyne-containing polymers can be derivatized in useful ways, for example by addition, hydrogenation, “click” reactions with azides, or ligation with various metals or catalyst entities. Electronically, alkyne linkages are useful and advantageous because the resulting products may be photoluminescent or, if suitably doped, electrically conducting. The alkyne-containing polymers of the present invention have particular light emitting properties and are in some cases fluorescent. This is useful because it allows applications, for example, as microporous fluorescent chemical sensors.
The conjugated microporous polymer has conjugated segments in its struts. Optionally, the struts may be conjugated through the majority of their length, optionally substantially all of their length. Optionally, the conjugation may extend through the majority of the node, optionally substantially the entire node. Optionally the entire network may be conjugated.
Any suitable palladium (0) catalyst may be used. For example, phosphine palladium (0) catalysts have been found to work well in the present invention. A preferred catalyst is tetrakis-(triphenylphosphine) palladium. Other suitable catalysts include tetrakis(methyldiphenylphosphine)palladium(0), bis(tri-tert-butylphosphine)palladium(0), bis-[1,2-bis(diphenylphosphino)ethane]palladium(0), and other similar catalysts.
The reaction is usually carried out at a temperature somewhere in the region of 0 to 300° C. The upper temperature limit may of course be limited by the reflux point of a solvent in which the reaction may be carried out. Other upper limits may be for example 200° C., 100° C. or 80° C. Other lower limits may be for example 15° C., 20° C., room temperature, 30° C., 40° C. or 50° C. Preferred temperature ranges include 30° C. to 100° C. and 50° C. to 80° C.
The reaction is usually carried out for a duration somewhere in the region of 1 hour to 150 hours. Other upper limits may be for example 120, 100 or 80 hours. Other lower limits may be for example 3, 6, 12, 24, 35, 50 or 60 hours. Preferred reaction times include 50 to 100 hours and 60 to 80 hours.
Preferably the reaction is carried out under inert atmosphere conditions, for example under nitrogen or argon. This is advantageous, inter alia to prevent homocoupling of alkyne monomers.
The reaction conditions may be varied so as to obtain optimal properties such as surface area properties.
Catalytic amounts of palladium (0) catalyst (and CuI if present) are used. Typically, 0.1 to 5 mol %, for example 1 to 5 mol %, have been found effective though other amounts could be used.
The microporous polymers made in accordance with the present invention are organic in the conventional sense that would be understood by the skilled person, and in contrast to inorganic polymers. In other words the element carbon is a key fundamental constituent thereof. The polymers also contain the element hydrogen and furthermore may also optionally comprise other elements, such as for example nitrogen, oxygen and various other elements.
The microporous polymers made in accordance with the present invention optionally have an average pore size of less than 10 nm, typically less than 2 nm.
From a further aspect the present invention provides a conjugated microporous polymer obtainable by a method according to the invention described herein.
From a further aspect the present invention provides a conjugated microporous polymer comprising:—
nodes comprising at least one aryl unit; and
struts comprising at least one alkyne unit and at least one aryl unit;
wherein a node is bonded from its aryl unit or units to at least two struts via alkyne units,
and wherein the conjugated microporous polymer has a halogen content of not more than 10% by mass.
A low halogen content is advantageous because this minimizes possible adverse environmental effects. It is important that minimal halogen or halogen compounds escape from the products over time. Furthermore, high halogen content results in unnecessarily heavy products (which is particularly undesirable, for example, in gas storage applications where lightweight materials are advantageous) and products which react unpredictably in further derivatization reactions. A key advantage of the present invention is the reliability and adaptability of being able to produce products of particular structure and properties.
Preferably the halogen content is no more than 9%, more preferably no more than 8%, more preferably no more than 7%, preferably no more than 6%, preferably no more than 5% by mass.
Other preferred features of the structure and composition of the conjugated microporous polymer are as discussed herein in relation to the method of preparation.
From a further aspect of the present invention provides a conjugated microporous polymer comprising:—
nodes comprising at least one aryl unit; and
struts comprising at least one alkyne units and at least one aryl unit;
wherein a node is bonded from its aryl unit or units to at least two struts via alkyne units, and wherein the conjugated microporous polymer has a BET surface area of at least 400 m2/g.
Thus it is possible, surprisingly, to combine the advantages of conjugation (fluorescence, chemical reactivity, electrical properties) with the properties of high surface areas and microporosity (of importance in, for example, areas such as molecular separations, high gas sorption, and catalytic activities) to produce materials with surprising and unprecedented combined properties.
Preferably the BET surface area is at least 450 m2/g, more preferably the BET surface area is more than 600 m2/g, most preferably the BET surface area is more than 750 m2/g.
Other preferred features of the structure and composition of the conjugated microporous polymer are as discussed herein in relation to the method of preparation.
FIGS. 1 to 33 and the features therein will now be described in more detail.